In some situations, it may be desirable to precisely determine the location of objects or devices that emit radio frequency energy such as radar pulses and communications signals. For example, military platforms, such as aeronautical vessels such airplanes, helicopters, and missiles as well as nautical vessels such as boats, often need to accurately locate threats that emit radio frequency energy. Working singly or in cooperation with another movable platform, a high precision radio frequency direction finding system can locate an RF emitter within a small enough volume of space to target and destroy the emitter with conventional or GPS guided weapons.
To target a threat emitter, it is desirable for military platforms to determine the location of a threat emitter within a volume smaller than a cube thirty meters (˜100 ft) on a side. This can only be accomplished if the military platform can measure the emitter's angle of arrival to less than 0.1 degrees and the military platform's attitude with respect to earth coordinates is known to less than 0.1 degrees in roll, pitch and yaw. Usually, the local wind, ocean waves and other effects introduce enough uncertainty of the military platform's attitude so that the threat emitter geo-location uncertainty is too large to allow it to be targeted.
Aircraft based radio frequency direction finding (RFDF) systems are used by the military to detect and geo-locate radio frequency emissions of radars, commercial broadcast transmitters, data links and other communications transmitters. As shown in FIG. 1, a fixed or slowly moving RF emitter 102 can be geo-located using triangulation. The triangulation process can be accomplished simultaneously by using multiple dispersed ground-based or aircraft-based direction finding receivers, represented by planes 104-108. The triangulation process can alternatively be accomplished by flying a single direction finding receiver on a single aircraft, shown at different position 104-108, by the RF emitter and taking multiple RFDF measurements. Different angles of the aircraft at different positions are shown as θ1-θ3.
The geo-location measurement accuracy is dependent upon the accuracy of the true angle-of-arrival (AoA) measurement and the accuracy of the geo-location of the direction finding receivers. For example, if the AoA azimuth accuracy is less than 0.1°, the cross range error is less than 360 feet at a range of 40 statute miles. Multiple AoA measurements from different directions will locate the emitter inside an ellipse with major and minor axes of less than 360 feet. This is accurate enough to target the RF emitter with a weapon or direct an electro-optic sensor to look at the location, where the RF emitter or people nearby can be identified.
Obtaining high AoA accuracy from a moving aircraft or another military vehicle has proven to be difficult. The aircraft attitude uncertainty and motion can be much greater than 0.1°. Also, the installed accuracy of a typical RFDF system is between 1° and 5°. For example, the local wind, ocean waves and other effects can introduce enough uncertainty of the military platform's attitude so that the threat emitter geo-location uncertainty is too large to allow it to be targeted. In addition, the signal-to-noise ratio (SNR) in the receivers will determine the angular resolution uncertainty and the final accuracy of the AoA measurement.
An aircraft's attitude can be measured with an inertial navigation system, however, only the most expensive inertial navigation systems have the resolution to measure aircraft attitudes to less than 0.1 degrees. Inertial navigation systems have positional drift with time and need periodic updates to realign their position and attitude. Because of this reason and others, typical methods used today to attack radio emitters include the use of radio frequency homing weapons, such as the high speed anti-radiation missile (HARM), which homes in on a RF emitter. Examples of such missiles include air-to-ground, surface-to-surface, surface-to-air, and air-to-air types. Such missiles are typically very expensive.